Commercially, sodium orthophosphates have traditionally been prepared as aqueous solutions, using either soda ash or aqueous sodium hydroxide as the alkali source. In some cases, such as in preparation of trisodium phosphate, both soda ash and sodium hydroxide are used sequentially. Where there is a mere choice between soda ash and sodium hydroxide, soda ash has generally been preferred because it is usually lower in price and overall cost of use than sodium hydroxide. Particularly in the manufacture of sodium tripolyphosphate, where energy costs for water evaporation are a significant economic factor, soda ash has been preferred, and processes have been developed for production of concentrated slurries of the orthophosphate precursor.
However, there has been a change in the method of soda ash manufacture and in the properties of the product. The Solvay process, which produces a fine, reactive soda ash, has almost completely been replaced in the United States by various processes which convert mineral deposits of "natural" sodium carbonate to a denser, more granular, less reactive soda ash. It has been believed that such granular natural soda ash must be fine milled to be useful in preparing sodium orthophosphate slurries suitable for sodium tripolyphosphate manufacture, U.S. Pat. No. 4,224,294 issued Sept. 23, 1980 to E. J. Powers. One reason for this belief has been the observation, in efforts to prepare sodium orthophosphate slurries, that a large proportion of the natural soda ash has remained unreacted, even after hours of boiling the slurry. It has been postulated that large particles of soda ash become coated with a layer of crystalline disodium phosphate dihydrate, Na.sub.2 HPO.sub.4.2Hz.sub.2 O, and are thereby rendered inert to further reaction.
There are many problems related to the milling of granular natural soda ash, among which are the cost of milling and special handling and equipment required, separate storage facilities, and the poor flow properties of fine-milled soda ash which cause difficulty in achieving uniform and consistent feed to processing equipment. These disadvantages are overcome by the present process which produces a concentrated, essentially carbonate-free, slurry of sodium orthophosphate using granular natural soda ash thereby avoiding the need to premill the natural soda ash. There is another problem related to the formation of a slurry suitable for the manufacture of pentasodium tripolyphosphate (STP). This problem results from the tendency of disodium orthophosphate, a constituent of STP precursor (a mixture of salts, principally disodium orthophosphate and the double salt whose chemical formula is Na.sub.2 H.sub.3 (PO.sub.4).sub.2), to form a dihydrate at temperatures below 95.degree. C. The dihydrate crystal robs the slurry of water, increasing its viscosity, and also tends to form large crystals, which reduces the slurry homogeneity necessary to produce high assay STP. The present process avoids dihydrate formation with the resultant increased viscosity, decreased homogeneity, and ultimately lower assay STP and is a further advancement in the art.